Support 2x resoruce in about_conflicts.html

BUG=None

Review URL: https://codereview.chromium.org/286933003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271568 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/about_conflicts.html

@@ -26,7 +26,11 @@ div#header {
 }
 
 #header h1 {
-  background: url('../../app/theme/default_100_percent/common/about_conflicts.png') 0px 20px no-repeat;
+  background-image: -webkit-image-set(
+      url('../../app/theme/default_100_percent/common/about_conflicts.png') 1x,
+      url('../../app/theme/default_200_percent/common/about_conflicts.png') 2x);
+  background-position: 0px 20px;
+  background-repeat: no-repeat;
   display: inline;
   margin: 0;
   padding-bottom: 43px;
@@ -35,7 +39,11 @@ div#header {
 }
 
 html[dir=rtl] #header h1 {
-  background: url('../../app/theme/default_100_percent/common/about_conflicts.png') right no-repeat;
+  background-image: -webkit-image-set(
+      url('../../app/theme/default_100_percent/common/about_conflicts.png') 1x,
+      url('../../app/theme/default_200_percent/common/about_conflicts.png') 2x);
+  background-position: right;
+  background-repeat: no-repeat;
   padding-right: 95px;
   padding-left: 0;
 }